Throughout this application, various publications are referenced by arabic numerals within parentheses. Full citation for these publications may be found at the end of the specification immediately preceding the claims. The disclosures of those publications in their entireties are hereby incorporated by reference into this application in order to more fully describe the state of the art as known to those skilled therein as of the date of the invention described and claimed.
Autocrine growth factors--factors produced by the target cells themselves--were first demonstrated for certain fibroblastic tumors in 1978 (1). Since then, autocrine growth factors have been described for several other tumors (2-5), and it has been recognized that normal cell such as activated T cells produce autocrine growth factors (6).
Blazer et al. (7) and Gordon et al. (8) reported that lymphoblastoid cell lines, (LCL) created by transformation with Epstein-Barr virus (EBV), make autocrine growth factors. The factors produced by one LCL stimulate growth of other LCL as well as of normal tonsillar B lymphocytes (9).
Buck et al. (10) described experiments to purify and characterize an autocrine growth factor produced by an EBV-transformed cell line. It was believed that the material was purified to apparent homogeneity and had no interleukin 1 activity.
Upon further study, the present inventors determined that the sequence of the protein was highly homologous to human prealbumin and that in fact bovine prealbumin had been isolated from fetal calf serum. It was also noted that the purified protein was not nearly as effective as a crude preparation, indicating the existence of a cofactor which was lost in later purification steps.
This cofactor was found to be a lipid, and through chromatography and other analysis it has been determined that the lipid is comprised of all-trans-retinol, commonly known as vitamin A. It has also been recognized that a second protein, retinol-binding protein, is contained in the growth factor.
Whereas neither prealbumin, retinol-binding protein or all-trans-retinol alone showed significant growth-promoting activity, the combination of the three did. This purified mixture possesses advantages over crude fetal calf serum because the former does not contain unknown components, is free from viruses and other hormones and growth factors and does not have the problem of impure antibodies containing calf serum for propagation of human or animal cell lines in cell culture.
The present invention, therefore, concerns the combination of prealbumin, retinol-binding protein and all-trans-retinol or other retinoids.